This pair of images shows how the northern polar ice cap on Mars grows and shrinks with the changing seasons on the Red Planet. The image on the left shows the polar region late in the northern spring season. Although the seasonal deposit of dry ice (frozen carbon dioxide) has receded from its maximum winter extent, it still covers a much larger area than during the warmer summertime. The right-hand image shows the much-reduced polar ice cap near the start of the summer season. Water ice, which remains frozen year-round, is the main constituent of the Residual Cap (or Permanent Cap) which can be seen in the image on the right. During the cold Martian winters, carbon dioxide gasfreezes out of the atmosphere as temperatures near the poles drop to -150° C (about -238° F). Although this seasonal dry ice layer is typically only a meter or two thick, its large geographic extent causes the wintertime size of the ice cap to grow substantially. In the spring, warming temperatures cause the dry ice to sublimate, returning it to the atmosphere. The southern polar cap on Mars also undergoes seasonal growth and shrinking. Likewise, Earth exhibits similar seasonal growth and retreat of its polar ice deposits. On our planet, floating sea ice (which forms a fairly thin layer atop the polar oceans) generates the greatest geographic variation in the extent of polar ice. Image courtesy of NASA/JPL/ Malin Space Science Systems.